The present invention concerns the field of integrated circuits and, more particularly, characterisation procedures for voltage converters connected to a capacitive circuit.
There exist a large variety of sensors as regards their shape, functions, effects and applications.
Within the scope of the detection of a parameter such as acceleration or pressure, a capacitive sensor such as that described hereinafter is commonly used.
FIG. 1 shows a conventional capacitive sensor 1.
Sensor 1 is arranged to be able to measure a parameter such as the ambient pressure or the acceleration undergone by such sensor. For this purpose, sensor 1 includes a support member 5, two plates 11 and 12 which are stationary relative to this support member, and a third plate 13 which is arranged so as to be able to move between the two plates 11 and 13.
An equivalent electric diagram of sensor 1 can be shown by two capacitances C1 and C2 connected in series. In such a diagram, capacitance C1 corresponds to the capacitance of the capacitor formed by plates 11 and 13, and capacitance 13 corresponds to the capacitance of the capacitor formed by plates 12 and 13.
Capacitive sensor 1 is also arranged so as to be able to provide a capacitance difference C1-C2 which is a function of said parameter. For this purpose, sensor C1 includes a connecting terminal 15 to be able to provide such a difference.
The operation of sensor 1 is as follows. Via the effect of said parameter, moving plate 13 moves in sensor 1, and this latter provides in response the capacitance difference C1-C2 which represents the arrangement of the three plates 11 to 13, following said effect.
Within the scope of the detection of a parameter such as pressure or acceleration, such a capacitive sensor is connected to a voltage converter, so that the circuit formed by the sensor and the converter, provides an electric voltage which represents the change in the arameter.
FIG. 2 shows such a known circuit including sensor 1 of FIG. 1, which is connected to a voltage converter 20.
Converter 20 includes two input terminals 201 and 202 and one output terminal 203. Converter 20 is connected and arranged so as to be able to receive, via terminal 201, the capacitance difference C1-C2 provided by sensor 1 and, via terminal 202, a bias voltage vb provided by a constant voltage source 22. Converter 20 is arranged so as to be able to provide, via terminal 203, an output voltage Vo which depends on the capacitance difference C1-C2 and the bias voltage Vb.
It will be noted that bias voltage Vb can be unipolar or bipolar, and referenced relative to the earth potential of the circuit. It will also be recalled that bias voltage Vb is typically used to fix the static gain of the circuit formed by sensor 1 and converter 20 at a predetermined value.
Generally, conventional characterisation procedures for such a converter, rely on the determination of the characteristic feature of output voltage Vo as a function of the capacitance difference C1-C2 and, in particular, the electric performance of this converter.
It will be recalled that the &lt;&lt;electric performance&gt;&gt; of a converter is usually characterised by two electric parameters: the static gain As and the non-linearity coefficient L.sub.As. It will also be recal dvb that the converter static gain As is equal to ##EQU1##
with reference to FIG. 2, and that non-linearity coefficient L.sub.As represents the dispersion of output voltage Vo between the effective values of this voltage and voltage values corresponding to ideal linear properties of the converter.
One difficulty commonly encountered in achieving such characterisation lies in the provision of a plurality of differences in capacitance C1-C2, to measure the change in output voltage Vo as a function of capacitance difference C1-C2.
Indeed, in the event that the converter is connected to a capacitive sensor such as that described in relation to FIG. 1, the variation in the parameter capable of causing a capacitance difference C1-C2 is difficult to control in an industrial manufacturing environment, in particular in semi-conductor manufacturing plants, which are subject to concerns as to yield.
In order to answer such concerns, a circuit, whose equivalent electric diagram is close to that of a capacitive sensor, i.e., a circuit which provides an electric signal capable of representing a capacitance difference, is used as capacitive sensor. In the following description, such a circuit is called a &lt;&lt;capacitive circuit&gt;&gt;.
By way of illustration, FIG. 3 shows a conventional capacitive circuit 25, capable of providing five capacitance difference values C1.sub.i -C2.sub.i i=1, 2, 3, 4, 5. For this purpose, capacitive circuit 25 comprises connection means 251 and five first capacitors C1.sub.i (i=1, 2, 3, 4, 5) respectively connected in series to five second capacitors C2.sub.i (i=1, 2, 3, 4, 5). Capacitive circuit 25 is arranged so as to be able to establish a connection between one of capacitors C1.sub.i, connection means 251 and the associated capacitor C2, so that this circuit provides, via connection means 251, one of the five capacitance difference values C1.sub.i -C2.sub.i.
One problem which is encountered in the determination of the electric performance of a voltage converter lies in the fact that such determination is unreliable, since the capacitance values are tainted by an intrinsic inaccuracy linked to the tolerance on the components and an extrinsic inaccuracy linked to the connection means of the capacitive circuit, these inaccuracies being all the more inconvenient if one desires to characterise a converter capable of processing low capacitance values.
FIG. 4a shows a curve 40 illustrating the voltage characteristic of converter 20 of FIG. 2, which is connected to capacitive circuit 25 of FIG. 3, and a curve 42 illustrating the linear regression of this characteristic.
It will be recalled that capacitive circuit 25 can provide successively five capacitance difference values C1.sub.i -C2.sub.i (i=1, 2, 3, 4 , 5). In the event that capacitive circuit 25 is used to simulate capacitive sensor 1 in the circuit of FIG. 2, five values for output voltage Vo can be obtained for the five respective capacitance difference values Vo.sub.i (i=1, 2, 3, 4, 5). In other words, five pairs of data items (C1.sub.i -C2.sub.i, Vo.sub.i) are thus obtained.
In order to determine the electric performance of converter 20, as described hereinbefore, these five pairs of data items are then extrapolated by a linear regression which is show in FIG. 4a by curve 42. This regression allows gain As, and coefficient L.sub.As to be determined.
It will be noted in FIG. 4a that the measurement error in output voltage Vo is essentially due to the error .epsilon. in the effective value of the differences in capacitance C1.sub.i -C2.sub.i.
For a sensor such as described in relation to FIG. 1, this sensor can provide capacitance differences C1-C2 which are less than several tens of femtofarads (1 fF=10.sup.-15 F), over a range of variation in capacitance difference C1-C2 which is typically comprised between several picofarads and several tens of picofarads (1 pF-10.sup.-12 F). For example, a converter having a 12 bit resolution allows a variation of 2.4 fF to be measured for a variation range of 10 pF.
FIG. 4b shows a theoretical curve 43 illustrating a relative accuracy designated .DELTA.Vo/Vo, which is linked to output voltage Vo obtained from FIG. 4a, as a function of capacitance difference C1-C2. It will be noted in FIG. 4b that the measuring accuracy of the electric performance of converter 20, determined by such a characterisation procedure, is tainted by error .epsilon. which is typically of the order of 1%, this value having been established by the Applicant of the present invention, by considering capacitances whose tolerances are of the order of 1%. It will be noted that this error is higher the lower the measured capacitance difference.
In other words, such a characterisation procedure does not answer current requirements as to accuracy and yield, which are common industrial concerns, in particular in semi-conductor manufacturing plants.